I recall that Lockwood Aviation sells used 912 and 914's pretty reasonable after they go through it. I recall they even provide a warranty. That's the safest approach to making your purchase low risk in my opinion. Plus they will have all updates on engine done.
But since I've never even heard of anyone getting a bad used 912, even buying from a private party is likely to be a low risk endeavor. The 912 is a very reliable and durable engine.